The present invention relates to an adhesive tape for temporary attachment, used during the production of ceramic electronic devices, e.g., multilayer ceramic capacitors, multilayer ceramic inductors, resistors, ferrites, sensor elements, thermistors, varistors, or piezoelectric ceramics, and in particular for use in a step of cutting a green (i.e., unprocessed) sheet of ceramic into a number of chips.
For example, a multilayer ceramic capacitor, as one example of a ceramic electronic device, may be produced in the following manner.
A slurry of ceramic powder is thinly spread out with a doctor blade to make a ceramic green sheet. After a plurality of electrodes are printed on the surface of the green sheet, a number of such green sheets are laminated and integrated so as to form a green sheet laminate. Next, after the laminate is initially heat-pressed, the laminate is cut in longitudinal and transverse directions with a cutting tool such as a dicer or a guillotine blade to obtain a number of ceramic laminate chips. These chips (also referred to as xe2x80x9cworksxe2x80x9d) are then sintered, and external electrodes are formed on end faces of the resultant works. Thus, a multilayer ceramic capacitor can be obtained.
In the aforementioned step of obtaining an integrated laminate and the step of cutting the laminate into green chips, a green sheet is temporarily fixed on a base for sheet stabilization by using an adhesive tape, and after the cutting, the works are peeled off the adhesive tape on the surface of the base.
As electronic apparatuses become downsized, the capacitance per unit volume of multilayer ceramic capacitors (which are electronic devices) must be increased. This has resulted in a trend for using increased numbers of laminated green sheets and harder ceramic materials. As a result, problems have emerged which were not associated with traditional green sheet cutting.
FIG. 1 illustrates a situation in which a laminate 3 is adhered to an adhesive tape 1 which is composed of a substrate film 6 and an adhesive layer 2 provided on the surface of the substrate film 6, the laminate 3 being about to be cut with a cutting blade. The adhesive tape 1 is affixed on a base 7. Electrodes 5 are buried at positions within the laminate 3 which shall define central portions, along a width direction, of the resultant chips.
For example, when the laminate 3 having a certain thickness is cut in predetermined sizes, cutting the laminate 3 along a cutting line X (solid line) in the figure causes the laminate 3 to be shifted along a lateral direction due to the pressure applied by the cutting blade, so that the broken section of the laminate 3 appears over at a transposed line Y. Thus, when a cross-section of the resultant chip 8 is observed after the cutting, the position of the electrode 5 within the chip 8 may be eccentric along the chip width direction, as shown in FIG. 2. This causes the following problems:
{circle around (1)} The eccentricity of electrode positions after the formation of the chips causes a degree of deviation in the distance between the electrode and an outer wall of each chip, this distance being a design factor required for each specific product. The distance (margin) m between the outer wall 8a and the electrode 5 greatly influences the puncture endurance of a capacitor. When there is eccentricity in the electrode positions, an interlayer destruction (referred to as a xe2x80x9cchip crackxe2x80x9d) may occur on the wall which is subjected to a reduced margin. This may cause malfunctioning of a ceramic capacitor.
{circle around (2)} Similarly, if the eccentricity of an electrode position within a chip exceeds the specified design tolerance, such a chip is considered as unacceptable, resulting in a decrease in the product yield.
The aforementioned phenomenon is presumably a result of a deformation stress applied at the time of inserting the cutting blade, which in turn is ascribable to the change in the material of green sheets.
Furthermore, when peeling the chips off the surface of an adhesive tape, it is necessary to reduce the adhesion strength between the chips and the adhesive tape. If the adhesion strength cannot be adequately reduced, the following problems may occur.
(1) Since the laminate itself has not been sintered yet, the adhesion between the respective layers may be insufficient. Therefore, when the chips are peeled off the surface of the adhesive tape, interlayer peeling may occur within the laminate due to an excessively strong adhesion strength of the adhesive tape.
(2) Even if interlayer peeling does not occur, the adhesive layer may be left as a contaminant on the bottom of a chip. As a result, when the chips are sent to a subsequent process, the contaminant may cause blocking. As the residue of the contaminant is also subjected to sintering, voids or cracks may occur due to the sintering of an organic material.
Thus, the reliability and the yield of the product are unfavorably affected.
In order to reduce the adhesion strength at the time of peeling the works, Japanese Publication for Opposition No. 6-79812 discloses an adhesive tape which includes an adhesive layer of a thermal foaming type, for example. A foaming agent is contained in the adhesive layer of this adhesive tape. As a result, by heating the adhesive tape after the cutting of a laminate, the foaming agent acts to reduce the area of contact between the adhesive tape and the works, so that the works can be easily peeled off the adhesive tape surface.
However, since the adhesive tape has a high foaming temperature, there are problems in that a binder within the laminate may evaporate to contaminate the works at the time of heating the adhesive tape, or that the laminate may fail to attain a predetermined hardness due to the evaporation of the binder prior to a preliminary sintering. Moreover, the adhesion strength may not be adequately reduced due to non-uniform foaming, so that the works may not be able to be peeled off the adhesive tape.
The temperature at which cutting is performed for an adhered green sheet is generally in the range of about 60xc2x0 C. to about 100xc2x0 C. However, the thermal foaming type adhesive sheet may begin a degree of foaming in a relatively high region (90xc2x0 C. or above) of this temperature range, so that the adhesive sheet may occasionally lose its adhesion strength. This may make it impossible to the affix the green sheet during the cutting process.
Needless to say, the aforementioned problem of eccentricity of the electrode positions within the green sheet associated with the cutting of green sheets has not been overcome.
An adhesive tape for temporary attachment of green sheets for a ceramic electronic device according to the present invention comprises a substrate film and an adhesive layer provided on at least one face of the substrate film, the adhesive layer comprising an adhesive composition, the adhesive composition containing a side-chain crystallizable polymer, wherein the side-chain crystallizable polymer comprises as a component an acrylic acid alkyl ester and/or a methacrylic acid alkyl ester which has a straight-chain alkyl group including 16 or more carbon atoms as a side chain, the side-chain crystallizable polymer having a first-order melting transition occurring in a temperature range narrower than about 35xc2x0 C., and wherein the adhesive layer has a modulus of elasticity of about 5xc3x97104 Pa to about 1xc3x97108 Pa.
In one embodiment, the side-chain crystallizable polymer is obtained from a monomer mixture comprising: about 40 to about 70 wt. % of an acrylic acid alkyl ester and/or a methacrylic acid alkyl ester which has an alkyl group including 1 to 6 carbon atoms; about 2 to about 10 wt. % of a carboxyl group-containing ethylenic unsaturated monomer; and about 20 to about 60 wt. % of an acrylic acid alkyl ester and/or a methacrylic acid alkyl ester which has an alkyl group including 16 to 22 carbon atoms.
A method for producing a ceramic electronic device according to the present invention comprises the steps of: allowing a green sheet laminate of ceramics to adhere to a base via an adhesive tape, and thereafter cutting the laminate into chips; and peeling the chips off a surface of the base while cooling the adhesive tape, wherein the adhesive tape comprises a substrate film and an adhesive layer provided on at least one face of the substrate film, the adhesive layer comprising an adhesive composition, the adhesive composition containing a side-chain crystallizable polymer, wherein the side-chain crystallizable polymer comprises as a component an acrylic acid ester and/or a methacrylic acid ester which has a straight-chain alkyl group including 16 or more carbon atoms as a side chain, the side-chain crystallizable polymer having a first-order melting transition occurring in a temperature range narrower than about 35xc2x0 C., and wherein the adhesive layer has a modulus of elasticity of about 5xc3x97104 Pa to about 1xc3x97108 Pa.
In one embodiment, the ceramic electronic device is a ceramic capacitor.
After a laminate which is obtained by laminating ceramic green sheets with electrodes printed thereon is heated, the laminate is affixed to a base via an adhesive tape, and thereafter the laminate is cut. Since the adhesive tape provides excellent tackiness, the laminate cannot be peeled at this time. When the works are removed from the adhesive tape after the laminate is cut, the adhesive tape is spontaneously cooled or forcibly cooled to a predetermined temperature or below so that the works can be easily peeled off the adhesive tape.
Since the adhesive layer has a modulus of elasticity of about 5xc3x97104 Pa to about 1xc3x97108 Pa, the lateral deformation stress, which is applied owing to the physical thickness of a cutting blade as it is inserted, can be cancelled or reduced by the elastic properties of the adhesive layer, thereby subduing the occurrence of shifts in the electrode positions.
As used herein, xe2x80x9cgreen sheets for ceramic electronic devicesxe2x80x9d encompass both green sheets and green sheet laminates composed essentially of ceramics, which are used during the production of ceramic electronic devices, e.g., multilayer ceramic capacitors, multilayer ceramic inductors, resistors, ferrites, sensor elements, thermistors, varistors, or piezoelectric ceramics.
According to the present invention, the tackiness of an adhesive tape for temporary attachment with respect to a laminate of green sheets of ceramics can be controlled simply by changing the temperature of the adhesive tape. Thus, a large adhesion strength can be obtained when temporarily attaching the laminate; when a work is to be removed, peeling can easily occur simply by cooling. Since the works will not be contaminated, it is possible to enhance the reliability of ceramic electronic devices such as multilayer capacitors.
Moreover, it is possible to attain cutting precision of chips (equalization of margins), thereby improving the reliability and yield of the products.